My opinion would be that you wouldn't need anything added to your water. There are many cases of people fasting for longer than 40 days on just water alone. My own longest fast was thirty days and I thrived on only a glass of water a day, working and doing strenuous exercise the whole time.
I believe that ANY serious deficiency in our system will result in a return of hunger and then you should eat for an equal amount of days raw foods (mostly if not solely) to replenish your stores before continuing. However if you start eating again before hunger returns you could continue your fast sooner.
As you say, what you suggest has been tried successfully before also. You are going outside the generally accepted parameters for a water-only fast, however experimenting can be a very good thing in fasting if you continue to listen to your body at all times and take all care possible. I do think it is far better to experiment within reason than to give up on the fast altogether.